On Thu, 8 Sep 2005, Matthias F. Brandstetter wrote:
SLR100" SCSI tape drive, which is capable of storing 50GB or 100GB (compressed) onto SLR tapes.
Note that 100Gb is compressed and the raw capacity is 50Gb.
Now I have a bunch of newly bought SLR tapes, but there I get an "out of space" error, it seems those new tapes can't save compressed data (= more than 50GB).
What did you expect? Assuming you have already compressed the data (or it is non-compressable data files), all hardware compression on the tape is likely to achieve is a slight increase in the total data volume.
How can I force hardware compression?
It's usually on by default with most hardware
Are there some types of tapes which aren't ready for hardware compression?
Probably, but what it really comes down to is that claims of 2:1 compression ratios are rarely seen in real world situations.
I get about 1.1-1.2:1 on full backups, and data incrementals, because the vast majority of what I'm backing up is either gzipped or incompressable formats such as jpg, mpeg or gif (astronomers and planetary physicists generate a LOT of imagary)
On system incrementals we see 6:1 or so, mainly because the vast majority of what's being backed up is extremely repetitive logfiles, however that only accounts for about 0.1% of total data backed up.
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